Resin-bonded abrasive articles such as grinding wheels are typically produced by blending discrete abrasive particles with a liquid binder material and a powdered resin, and then pressing the mixture under appropriate thermal conditions. Other constituents can be included in the mixtures, e.g., fillers, curing agents, wetting agents, and various metal powders. An aging period which allows for salvation of the dry portion of the mixture with the liquid resin is usually required before pressing.
As noted in U.S. Pat. No. 4,918,116 (Gardziella et al), novolac resins have often been used as solutions in solvent-based systems for binding abrasive articles. A disadvantage of such a system is the easy ignitability of the solvents at high temperatures. While solvent-free modified novolacs have been developed, these materials are quite expensive, and their commercial use may often not be economical.
In addition to the difficulties involved will using certain novolac binders for making abrasive articles, manufacturers are sometimes faced with other production problems as well. For example, as pointed out in Gardziella, the use of liquid grain wetting agents such as furfural when preparing molding materials for abrasive discs may result in an unstable molding mixture. Furthermore, the use of such a mixture may generate a large amount of dust--often a drawback on the manufacturing floor.
The dust and stability problems associated with using novolac binders appears to have been somewhat alleviated by the teachings set forth in Gardziella. This reference discloses the preparation of various molding materials, using specific phenol-novolacs having a phenol-formaldehyde molar ratio of 1:0.2 to 1:0.35. As an example, abrasive discs are prepared by using heated corundum grains wetted with a not melt of the specified phenol-novolacs. After being blended at 140.degree. C. in a high-power mixer, the composition is cooled to 90.degree. C. and then further blended with a second novolac resin and a curing agent.
The type of novolac resins discussed in Gardziella may be used in a multi-step process for preparing a precursor molding material for abrasive articles. This process involves applying a heated, liquid novolac resin to abrasive particles by a mixing technique, followed by the application of a dry bonding material. The dry material usually includes another novolac resin, fillers, and one or more curing agents. The sequence of applying liquid resin and dry binding material is repeated in two or more discrete steps to eventually obtain dry, flowable, homogeneously-coated abrasive grains.
From this brief discussion, it should be apparent that there are advantages and disadvantages involved in using various novolac resins to prepare articles like abrasive discs. A process which does not require high-power mixers and temperatures as high as 140.degree. C. is commercially attractive. Additional economic benefits result from a continuous process which does not require multiple applications of liquid binder and dry binder, and which does not call for the heating and cooling cycles described in Gardziella. Moreover, elimination of the time-consuming aging step before pressing is another major improvement.
Ideally, these process improvements would not occur at the expense of material properties. For example, the attainment of flowable, homogeneously-coated abrasive grains (which are also substantially dust-free) as a molding material for abrasive wheels is an important objective in a manufacturing setting.
The homogeneous surface coating on the abrasive grain of the molding material of this invention causes substantially all abrasive grain to be uniformly distributed through the abrasive article prepared from the molding material. The abrasive article, both in its green and its cured state, is substantially free of bond rich regions, providing a uniform distribution of abrasive grains throughout the cross-section of the abrasive article. Because the abrasive grain is uniformly distributed, an abrasive wheel made from the molding mix is balanced. In traditional manufacturing processes, the abrasive wheel must be balanced by removing out a portion or the side or the wheel so that the wheel is true and spins in a balanced fashion during grinding operation. Many wheels which are out of balance are rejected, causing production losses. The elimination of all or some of the balancing steps from the manufacturing process is a significant improvement. In addition, the uniform distribution of abrasive grain provides better grinding performance and longer wheel life during use.
Finally, the characteristics of the abrasive article should remain very attractive. In the case of an abrasive wheel, the desirable properties include grindability and long working life.